The present disclosure relates to a signal transmission cable and a flexible printed board, and more particularly to a signal transmission cable and a flexible printed board, by which a low loss, space-saving parallel transmission path can be provided.
In recent years, for example, there are more and more needs for increasing the speed and volume of data communication in electronic apparatuses such as a smart phone. Correspondingly, a signal frequency is becoming higher, for example, a range of from several GHz to several tens of GHz.
Also, in order to increase the signal rate, transmission paths are arranged in parallel and the number of channels is increased. Currently, for this purpose, thin coaxial parallel cables in which several to several tens of micro lines called thin coaxial lines are arranged in parallel are widely used.
However, even with such thin coaxial cables, in a frequency range of 20 GHz or higher, the dielectric loss due to the dielectric substance is increased, which deteriorates the cable properties.
For example, a metal waveguide is used from the past as a low-loss transmission path for a microwave band or a millimeter waveband. The metal waveguide has a rectangular or circular tubular hollow structure. The dielectric substance that causes the dielectric loss is the air, and hence the metal waveguide is characterized by extremely low loss.
However, it is difficult to arrange metal waveguides in parallel and reduce the weight thereof due to their structure. Further, the cost is high and a flexibility cannot be provided. Thus, there is a problem that the metal waveguides cannot be used as the parallel transmission paths in an electronic apparatus. Further, there is proposed a high-frequency flexible multiconductor cable connecting system including a structure similar to the coaxial structure is embedded in a film of a dielectric substance (for example, see Japanese Patent Application Laid-open No. 2003-203694 (hereinafter, referred to as Patent Document 1). In the technique of Patent Document 1, after cables of a rectangular coaxial structure having central conductors, through which signals are transmitted, is surrounded with an insulating material, and the insulating material is further coated with an external conductor are formed, a plurality of such cables are bundled in parallel, whereby means for achieving high-speed transmission and improvement of anti-noise characteristic are provided. At a fitting portion for connecting the multiconductor cable to a control circuit, the central conductor of the cable portion is projected. On the contrary, the central conductor is recessed at a cable fitting portion. Alternatively, their structures are converted, whereby the fitting portions are connected in close contact to retain the continuity of line impedance matching.
In addition, in recent years, there is proposed a dielectric waveguide that forms a structure of a waveguide embedded in a multi-layer wiring substrate with a dielectric substance.
This dielectric waveguide is also called a substrate integrated waveguide (SIW). The dielectric substance is sandwiched between two conductors and a plurality of via-holes connecting between the two conductors are arranged in two columns. In this manner, the dielectric waveguide performs a signal transmission at the same transmission mode as the metal waveguide. This dielectric waveguide is capable of performing a lower-loss transmission in comparison with the coaxial lines and is suitable for transmitting a signal having a frequency higher than several tens of GHz.